We have used a batch type microcalorimeter to show that the substrate L-Glutamate and the feedback inhibitor L-Alanine bind to separate sites on the enzyme glutamine synthetase on the basis of additive heats of binding. Colorimetric assays have been developed for glutamine synthetase adenylyltransferase, ATase, and the regulation protein, PII. These assays are based upon ATase and PII-stimulated adenylylation or deadenylylation of glutamine synthetase.